1. Field of the Invention
The present invention relates to a lithographic apparatus, and an apparatus and method for measuring a position of an object in a medium.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The patterning device may be held on a movable patterning support (also referred to as a reticle stage), and the substrate may be held on a movable substrate support (also referred to as a wafer stage). The patterning support and the substrate support each are moved by one or more motors that are accurately controlled by a positioning system. To enable control of the patterning support and the substrate support with a high accuracy, typically in the order of nanometers, the positioning system includes a laser interferometer system to determine a position of an object very accurately.
In a laser interferometer system, which is an incremental system where the wavelength of the laser light used constitutes a measurement unit, a reflection of a laser beam against a reflecting surface of the object is measured and compared with an internal reference path. If the object position changes in line with the laser beam, the comparison between the reference and the measuring laser beam will show a number of interference transitions (consisting of constructive/destructive interference between the measuring path and the reference path) which is proportional to the displacement. The number of interference transitions is not only proportional to the displacement, but also with the wavelength of the laser light. More particularly, the total amount of interference transitions is equal to the total optical path divided by the wavelength (in that optical path) at the end of the displacement minus the total optical path divided by the wavelength at the start of the displacement. When the total optical path at the end of the displacement is regarded as a sum of a constant optical path (=the total optical path at the start of the displacement) and a displacement optical path, then the total amount of interference transitions includes the displacement optical path divided by the latest wavelength and the constant optical path divided by a changed wavelength (the constant optical path divided by the wavelength at the end of the displacement minus the constant optical path divided by the wavelength at the start of the displacement (see also formula [4] below)).
The wavelength of light in air depends on the nominal wavelength in vacuum, divided by the refractive index of the medium through which the light travels. The refractive index in air nair may be described by the so-called Edlen formula (B. Edlen, The Refractive Index of Air, Metrologia, Vol. 2, No. 2, pp. 71–80 (1966)).
Another version of the Edlen formula, in which the basic interdependencies of the parameters are unchanged, is described in K. P. Birch, M. J. Downs, 1994, Correction of the updated Edlen equation for the refractive index of air, Metrologia 31, pp. 315–316 (included herein by reference).
                                          n            air                    -          1                =                                            D•P              96095.43                        ⁢            •            ⁢                                          1                +                                                      10                                          -                      8                                                        ⁢                                      (                                          0.601                      -                                              0.00972                        ⁢                                                  •                          ⁡                                                      (                                                          T                              -                              273.15                                                        )                                                                                                                )                                    ⁢                  •P                                                            1                +                                  0.0036610                  ⁢                                      •                    ⁡                                          (                                              T                        -                        273.15                                            )                                                                                                    -                      f•3            ⁢            .63442            ⁢                          •10                              -                10                                                                        [        1        ]            
which is valid for λ=633 nm and concentration CO2=450 ppm, and wherein:
D=2.7653·10−4 
T=absolute temperature [K]
P=pressure [Pa]
f=humidity [Pa]
It will be appreciated that nair, according to formula [1], depends on the pressure of the air as well as on other parameters. In view of the above, when measuring a position with a laser interferometer system, it is desirable to at least take into account the pressure of the air. When the pressure is accurately known, the refractive index may be accurately determined. Further, the number of interference transitions (also termed “fringe count”) may be accurately determined in the laser interferometer system. Combining the results of the pressure determination and the fringe count, the position sought may be accurately and repeatably determined.
Generally, it results from the foregoing discussion that, in order to accurately measure an object displacement in a medium, such as air, using a laser interferometer system or any other measurement system based on the determination of a number of wavelengths, it is desirable to accurately determine the pressure of the medium.
A pressure is measured by a barometer, preferably an absolute barometer. On the one hand, a barometer may be capable of a continuous pressure measurement. On the other hand, a barometer may be an electronic barometer, having a limited update rate in the order of tens of Hz in view of the averaging time needed for the required accuracy. For different kinds of barometers, account has to be taken of a certain measurement delay which for electronic barometers may include half an averaging time, and overhead introduced by the measurement system. This introduces errors in the measurements of pressure changes, which results in errors in the position measurements based on laser interferometer position information, which position measurements in themselves are sensitive to pressure changes.
A disturbance leading to a relatively rapid, possibly periodic pressure change may cause an error which cannot be followed by the barometer.
One might seek to overcome this problem by compensating a (patterning or substrate) support position measurement using a reference measurement, such as a measurement of the position of an optical component (e.g., a lens). In the compensation process, a reference distance is compared with an actual distance corresponding to a support position, assuming a good correlation between the refractive index of both optical paths. However, since the actual distance may differ substantially from the reference distance, the performance of the compensation process will degrade when the difference between the actual distance and the reference distance increases.
Further, it is to be noted that the mechanical components (e.g., a metrology frame, a lens) supporting parts of the laser interferometer system will show resonances, and show dimensional drift when temperature changes. Such effects may also deteriorate the accuracy of a measurement of a support position or displacement, if no compensation takes place.